The present invention relates to electromagnetic position sensors generally. The invention has particular, although not exclusive relevance to inductive position sensors for use in a piston and cylinder assembly.
Several conventional sensing techniques are currently used for industrial cylinder position measurement. These include linear variable differential transformer (LVDT) systems, magnetostrictive time of flight systems and optical position sensors. The main problem with these existing systems is that they are relatively expensive (more than $100) to install in a hydraulic or pneumatic cylinder. Additionally, a particular disadvantage of the LVDT sensor is that the sensor body must be longer than the maximum stroke and therefore, incorporating this sensor into a hydraulic or pneumatic actuator usually involves a significant amount of redesign of the cylinder including the attachment of an additional member onto the piston. Another problem with the LVDT system is that it is relatively sensitive to alignment between the excitation and sense coil and to temperature variations of the cylinder.
One aim of the present invention is to provide an alternative sensing system for use in a piston and cylinder assembly which is relatively simple in operation and which can be produced at low cost and can be easily integrated into the cylinder without significant modification to the mechanical design.
According to one aspect, the present invention provides a position sensor comprising: an excitation winding, a sensor winding, a compensation winding and an electromagnetic field modifying member; wherein the field modifying member and the windings are relatively movable over a measurement path and are arranged so that the electromagnetic coupling between the excitation winding and the sensor winding varies as a function of the relative position of the field modifying member and the windings; and arranged so that the electromagnetic coupling between the excitation winding and the compensation winding does not vary with the relative position between the field modifying member and the windings. In this way, the signal output from the compensation winding can be used to compensate for variations in the signal output from the sensor winding due to, for example, temperature variations in the position sensor system. The output from the compensation winding can be used in addition or alternatively to stabilise an excitation signal which is applied to the excitation winding.
Preferably, the windings are substantially planar and the modifying member is mounted relative to the windings so that there is rotational symmetry between the windings and the modifying member. In this way, rotation of the field modifying member relative to the windings does not cause a variation in the output signal levels. In a preferred embodiment, the windings are formed by copper tracks formed on a number of layers of printed circuit board, since PCB techniques allow accurate alignment and positioning of the windings relative to each other and relative to the field modifying member.
According to another aspect, the present invention provides a position sensor comprising: an electromagnetic field modifying member; a winding electromagnetically coupled to said field modifying member, for generating an electromagnetic field; and drive means for applying an excitation signal to said winding; wherein said field modifying member and said winding are relatively movable along a measurement direction between a first position and a second position and wherein said drive means is arranged to apply an excitation signal having a frequency that minimises a change in coupling between eddy currents generated in the field modifying member and the winding between the first and second positions for different operating temperatures.